The present invention relates to master cylinder assemblies for hydraulic control, in particular for control of the braking of a vehicle.
A master cylinder assembly for hydraulic control is known from FR 2134082 of the type comprising:                a master cylinder body in which is formed a longitudinal bore emerging at the level of one end of the said master cylinder body,        at least one piston arranged in the said bore and provided, on the one hand, with a seal packing which is in tight contact with a peripheral wall of the said bore and which separates a pressure chamber and a supply chamber intended to contain a liquid in the said bore and, on the other, a recovery conduit connecting the said chambers by-passing the said seal packing,        a valve moveably carried by the said piston and elastically biased towards a position closing the said recovery conduit,        a stop device retained longitudinally in the said bore and able to co-operate with the said valve in a predetermined rest position of the said piston to define a corresponding open rest position of the said valve.        
In this master cylinder assembly, the stop device must open the recovery conduit in the rest position of the piston. The piston necessarily has, departing from the rest position, a certain dead travel in the course of which substantially no hydraulic control pressure is created in the pressure chamber, due to the fact that the recovery conduit is not closed and the liquid can therefore escape into the supply chamber through the recovery conduit. This dead travel corresponds to closing travel of the moveable valve carried by the piston.
The stop device of this master cylinder assembly is adjustable, which, on building of the master cylinder assembly, permits precise adjustment of this dead travel to meet two requirements. On the one hand, it is desirable to minimise this dead travel in order to improve the response time of the master cylinder assembly, which is particularly critical to the efficiency of a braking system (dead travel in the master cylinder resulting in a distance travelled by the vehicle unbraked) and, where necessary, in order to limit the travel of the brake pedal for the comfort of the user. On the other hand, it must ensure satisfactory opening of the recovery conduit when the piston returns to its rest position, in order to ensure that the pressure in the pressure chamber drops back to a reference pressure, typically atmospheric pressure, which is maintained in the supply chamber by means of a supply conduit. The opening of the recovery conduit is also necessary so that when the hydraulic circuit at the output of the master cylinder assembly has a leak, the level of liquid in the supply reservoir drops, which permits detection of the leak.
In this device, even with very fine adjustment it is therefore impossible to completely eliminate the dead travel of the piston from its rest position. This device therefore does not offer maximum safety or optimal user comfort as a result of an inescapable response delay due to this dead travel.
The aim of the invention is to propose a master cylinder assembly not having at least certain of the above-mentioned disadvantages of the prior art. More precisely, the aim of the invention is to propose a master cylinder assembly in which the response time is improved.